1. Field of the Invention
The invention relates to the field of ultrasonic (including megasonic) cleaning of substrate surfaces.
2. Description of Related Art
Removal of particulate contaminants from a semiconductor substrate can be accomplished by ultrasonic cleaning. When the frequency of ultrasound is close to or above 1,000 kHz (1 MHz) it is often referred to as “megasonic”.
Acoustically activated bubbles close to any liquid-surface interface causes (a) shear stress at the surface, which can lead to the removal of particulate contaminants from the surface, (b) microstreaming, which can lead to the enhancement of diffusion limited reactions beneficial for electrochemical deposition processes, etching, rinsing and mixing, and (c) local enrichment of active components close to the surface such as free radicals, ozone and plasma, to impact chemical processes such as oxidation processes and etching.
Controlling the behavior of bubbles within a sound field is essential for any cavitation driven process. Furthermore, if cavitation should be spatially controlled, such as bubble arrangements and bubble activity at a solid-liquid interface during e.g. ultrasonic cleaning, the design of a specific sound field is required. This can be either done by an engineered acoustic near field interference pattern or a standing wave pattern, that can be achieved by a multiple resonator setup, a structured resonator, or a resonator stack parallel to the substrate.
The preferred way to obtain spatially controlled cavitation is the use of structured resonators. However, conventional structured resonators require the presence of a structured surface, which leads to 1) an increased liquid volume between resonator and substrate to fill up this structure (compared to a resonator stack parallel to the substrate) and this leads to an increased consumption of process chemicals, 2) the presence of bubbles, which may typically adhere on the structure and are difficult to remove or give rise to wetting issues within these structures.
The presence of adhered bubbles and wetting issues will both have an enormous impact on the efficient transmission of sound waves and the effective sound field in the liquid. In prior art solutions for e.g. cleaning by ultrasound it is the aim to produce a volume of high acoustic energy where cavitation is the basis of the cleaning effect. In such solutions the cavitation and with this the bubble movement weren't directly controlled by the resonator that produces the ultrasound. Prior art solutions where often based on tanks which are sonicated by low or high frequency ultrasound transducers (“Megasonic Cleaning” e.g. U.S. Pat. No. 6,148,833). Such solutions allow batch processes of single or groups of wafers.
There are also single wafer processes chambers, in which the transducers are located close to the wafers surface (e.g. WO0021692). Also here, cavitation is not spatially controlled. The aim is only to produce a high acoustic intensity in the process liquid.
A solution to clean a small region of a wafer is to use a transducer with a rod to transmit the sound waves (e.g. U.S. Pat. No. 6,039,059). From this rod, sound can be emitted to the process liquid to clean the wafer. Also with such a solution there is no direct control of bubble movement on the surface to be cleaned.
Therefore, it would be beneficial to integrate a structured resonator in a flat resonator block but in such a way that it does not require distance optimization to guarantee sufficient power transmission to the processing liquid as is required for many conventional resonator stacks that are positioned parallel to the substrate.